This invention relates to a water based aluminum paint for plastic film, especially for outdoor agricultural uses.
Farm growers often use a polyethylene film around growing plants, especially of the vine type, to conserve the moisture in the soil and to reduce the growth of weeds. The film also holds in the nutrients and acts as a mulch. This practice is generally useful in the tropical and sub-tropical areas because of the intense rays of the sun.
We are concerned primarily with the large growers with an average area of 500 acres of bush or vine-type vegetables and fruits, such as tomatoes, peppers and melons. In the tropical and sub-tropical zones, because of the intense heat of the sun's rays, the farmers, using various plain films and films painted white, learned that it was necessary for the film to have a reflective quality so as to maintain the moisture and nutrients in the soil. Clear polyethylene film is too expensive because it is made from pure pellets of polyethylene which require a higher machine temperature to melt to a viscous liquid form so that it can be properly extruded into films. Black polyethylene, which is made primarily of scrap film, does not require this high heat and thus the film can be prepared at a higher speed and consequently lower cost.
The plain or unpainted film did not solve the farmer's problem as the film allowed the sun's rays to penetrate through the film. However, the black film absorbed the sun's rays. When painted with a white paint, the film had only a small amount of reflectivity and acted primarily as an insulator.
A solvent based aluminum paint was then used, and found to give the desired reflectivity when applied to a black colored polyethylene film. However, in order to reduce the viscosity so that the paint would be suitable for spray painting, mineral spirits from petroleum was added. This was expensive, especially since this solvent contributes no other value. In addition, mineral spirits has an offensive odor and pollutes the atmosphere. Also the equipment must be washed with a solvent which increases the cost, requires more labor and more exposure of the laborer to the solvent.
Owners or growers refuse now to use the solvent based aluminum paint because of the health and fire dangers and the high cost of solvents. Because of the energy situation this cost will continue to increase and become more difficult to obtain.
The paint for the plastic film requires the essential reflective characteristics. Other requirements should also be met. For instance, it is desirable that the painted film have a degree of water resistance. A degree of flexibility of the painted film is important also so that the paint will not rupture or craze and as a result be loosened to the extent that it can be removed by the elements such as rain, and/or wind.